Known thread brakes of this type are highly prone to soiling, since fluff and lint or residues of oil, paraffin or similar adhering to the threads easily accumulate on the thread contact surfaces or brake surfaces during operation. Therefore, it is known to rotatably dispose the brake elements on a bearing pin (DE 35 04 739 A1) or combine the bearing pin, the brake elements and the usual pretensioning mechanism intended for adjustment of the braking force to form one structural unit disposed to be freely rotatable as a unit in the holding means (DE 197 20 795 A1). The purpose of the rotatable mounting of the brake elements and/or the entire structural unit is to use the tangential forces exerted on the brake elements when the threads are moving to set the brake elements in rotation, and thus cause constant self-cleaning of the brake elements, in particular if their walls are provided preferably with adequately large openings.
In addition, thread brakes are known, in which the brake elements are coupled to a device that generates vibrations (DE 41 04 663 C1) such that a force action promoting the rotational movement of the brake elements results. However, such additional devices in machines, which use or process a large number of threads, are not acceptable from an economic viewpoint, since they involve enormous structural expense.
Apart from this, it has been found that, despite application of the above-described measures, fluff and lint adheres in particular to the pretensioning mechanisms for the brake elements. The pretensioning mechanisms are provided with helical pressure springs, and as a result thereof, the desired braking force can vary during operation of a circular knitting machine or similar, or at least a reproducible adjustment of the braking force can be made more difficult. This leads to frequent cleaning operations and thus to outage times of the machines. To avoid this disadvantage, protective caps are known (DE 41 12 898 A1), which are attached to the pretensioning mechanism and cover the helical pressure springs. However, such protective caps not only increase the structural expense, but also hinder access to the brake elements and their self-cleaning. Known thread brakes, in which the braking force is generated or released with gas pressure (DE 36 29 928 C2), are also associated with high expenditure.
Moreover, it is known (DE 43 01 507 C2, 295 21 428 U1) to adjust the braking force with the assistance of permanent magnets instead of helical pressure springs. The advantage of reduced capacity for dirt to accumulate on the pretensioning mechanisms is offset here by the disadvantage that the pretensioning force of the brake elements, and therefore the braking force exerted on the thread, is only variable by replacing magnetic inserts, which is associated with high time expenditure, and only in comparatively large steps, whereas the springs provided in conventional thread brakes allow continuous adjustment of the braking force. Therefore, the thread brakes used in practice, at least in circular knitting machines, contain pretensioning mechanisms, which are provided with open springs without protective covers, so that despite there being a tendency towards self-cleaning, they have to be cleaned in relatively short time intervals, and in a machine that processes or uses threads with a plurality of thread guides, e.g. a circular knitting machine with 96 systems or more, this necessitates expensive cleaning operations with corresponding machine outage times.